A video signal is made up of multiple frames with each frame being made up of multiple horizontal lines of image information. The video signal can be very sensitive to errors in phase as well as frequency. Collectively, the phase error and the frequency error can be referred to as time base errors. The time base errors can manifest in a video image in the form of image tearing, image noise, chromatic distortion, image distortion, overall image instability, and so on. The time base errors can be seen as differences in duration and start/stop times for each horizontal line. It is possible to correct the time base error in a video signal using analog signal processing or digital signal processing techniques. A prior art analog error correction technique may involve the use of circuitry containing various delay lines that are used to alter the timing of the video signal. A prior art digital error correction technique can make use of an analog-to-digital converter (ADC) to convert the video signal into a digital stream and a digital-to-analog converter (DAC) to convert the digital stream back into an analog signal, with timing correction being implemented via adjustments to clocks controlling the operation of the ADC and/or DAC.
In situations wherein an analog video signal has been digitized using a sampling clock with a sampling frequency, fS, any time base error present in the video signal can result in a differing number of samples in each horizontal line of the video signal as well as horizontal line start/stop times that differ from expected start/stop times. This is due to a sampling clock that remains constant while the analog video signal may contain time base errors.
In U.S. Pat. No. 5,600,379 (Wagner, issued Feb. 4, 1997), which is herein incorporated by reference, a technique for digitally correcting time base errors is presented using a sampling clock that is asynchronous with respect to a horizontal sync signal present in the video signal. A digitized version of the video signal is resampled using a sampling clock that resamples the digitized video signal by a specified factor that results in approximately 500 to 1200 resampled samples per horizontal line. The resampling can be performed using a running average of the horizontal sync frequency over a specified number of horizontal lines.
One disadvantage of the prior art is that the correction of errors in the time base occurs gradually over a number of periods. This gradual correction can result in errors remaining in the video signal longer than necessary and negatively impacting video images for an extended amount of time. A single large time base error can take a large number of periods to correct.
A second disadvantage of the prior art is that the use of the running average can mask out the effect of large time base errors if they occur relatively infrequently or if there are complementary time base errors. The prior art technique may not be able to detect the occurrence of the error and permit the time base error to remain in the video signal.